Bench to Bucket Conversion Advice

They are to prevent the nut from pulling through the sheet metal when you crash. The top load is spread out over a larger span of sheetmetal by the seat tracks themselves. the bottom gets the plates to spread the load from pulling up. If you are not concerned about factory look, go to Home Depot, Lowes, a scrap metal place, or look around your garage or work etc & find 4 plates of 3/16 or 1/4, approx. 3" square, Drill a hole in the center and use it as a big washer. At some point in the future if you get a welder, get some other work done, have a buddy, whatever- you can weld them in at that point.
I used some stock from HD that was about 2" wide, but it was like "U" channel, but a very shallow u. More like flat bar with the 2 edges turned up. I cut longer lengths and spanned 2 bolts. I think I went side to side, but I was welding subframe connectors and also welded to that. If there isn't any dip down to the floorpan, you might be able to span front to back. just a couple options.

That all said, I drove mine around for years with nothing but a standard size washer. back then, I never even knew a plate went there, and I wasn't wise enough to think about it any further than that.

EDIT, reading other posts further down, there might be a plate under the carpet, over the floorpan, If that's the case, I would still just use something I could find.